Conversion of coal, char, or coke to a gaseous product by reaction with heated air, oxygen, steam, or carbon dioxide or mixtures of these comprise the well-known process called coal gasification. The product of coal gasification is a mixture containing hydrogen and carbon monoxide and varying amounts of nitrogen, carbon dioxide, steam, hydrogen sulfide, organic sulfur compounds, and possibly tar and dust, depending on the gasification process and reactants employed.
Processes have been investigated for coal gasification at atmospheric and elevated pressures for nearly a century, but many problems still remain that inhibit efficient gas production. There is a tendency for coal to form a sticky agglomerating surface; the result of the tar and asphalt fractions of coal forming exudates when the coal is heated, and gasifiers that utilize coals with high free swelling indexes have been plagued with problems of coal clumping and caking.
Since the typical gasifier is a counterflow device, whereby the coal flows downward under the force of gravity, while air and steam used to heat and gasify the coal move upward through the coal bed, raw coal is fed to a relatively cool zone in the upper portion of the gasifier near where coal gases and other volatiles leave the device. This aggravates the swelling and caking characteristics of coal.
The conventional Lurgi pressure gasifier comprises a pressure vessel having at its upper end a coal lock hopper through which sized coal is introduced under pressure. The coal bed is stirred at its upper end to maintain porosity of the devolatilization zone and to break up any forming agglomerates. The coal is heated to between 1800.degree. F. and 2300.degree. F. by adding heated steam and oxygen at the lower end of the pressure vessel. A rotating grate is disposed within the lower end of the pressure vessel and is charged with coal to be gasified. The Lurgi gasifier has a major disadvantage in that the raw gas outlet is very near the top of the coal bed so that tars and fine grained dust and coal can be entrained within the exhaust stream of the gasifier.
The Morgantown Energy Technology Center (METC) gasifier is based on a similar concept, in which the coal is introduced at the top and air/steam are introduced at the bottom in a countercurrent manner. Typically, a pressurized lock hopper supplies coal to a variable speed rotary feeder which dispenses coal to a screw feeder. The screw feeder runs at a constant speed which is fast enough so that it always moves the coal into the volume of the pressure vessel before it can be hot enough to become sticky and initiate blockage. A water cooled, hydraulically driven, three blade stirrer is used to maintain bed porosity and provide a capability to utilize strongly caking coals. As with the Lurgi gasifier, tars and fines exist in the product gas. More importantly, the METC design requires a deep bed stirring function. This imposes complex forces on the bearing and pressure seal design for the water cool shaft which both slowly rotates and translates as it penetrates the gasifier pressure vessel.
Use of highly caking coals in these conventional coal gasifiers results in decreased throughput and therefore gasification output is drastically reduced at times to less than half the full load capacity. This is the direct result of the sticky, swelling coal which forces the gas path to short circuit through cracks within the coal bed. This short-circuiting is called "channeling" and is a significant problem because it ruins the necessary gas to coal contact and interaction that are needed to carry out efficient gasification reactions.
If the heating of coal could be performed at temperatures high enough to hasten the transient time through the gasification device, the swelling process itself could be limited even for highly caking coals. Therefore, it would be useful to provide a coal gasification process which allows the coal to become devolatilized by heating without agglomeration and subsequent channeling.